QOpenGLTextureHelper: add a clarifying comment

Change-Id: Ia27fd6616b2259b3fbe71ea8737b270c323a1256
Reviewed-by: James Turner <james.turner@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
QOpenGLTextureHelper: fix 3D texture support on ES2

The code first tried to check for the GL_OES_texture_3D extension,
and if present it resolved the pointers to the gl*Tex*3DOES functions.
But then it overwrote those pointers by attempting to resolve the
Desktop GL gl*Tex*3D functions, thus making them unusable.

So, if the extension is found, don't overwrite the pointers.
If the extension is NOT found, keep the general behavior of still
trying to resolve those functions.

(That is going to fail, but refactoring the general behavior
belongs to a separate commit.)

Change-Id: Idaba122cf9500f136e3f79284d3c82284257036d
Reviewed-by: James Turner <james.turner@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
QOpenGLTexture: fix support for 1D textures

OpenGL ES 2 doesn't support 1D textures. So introduce a proper
feature flag and warn if we try to allocate one there.

Change-Id: I73cf58c1f257d2472564f45bff222231e39aca52
Reviewed-by: James Turner <james.turner@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
QOpenGLTextureHelper: explain why we use GetProcAddress on Windows / DesktopGL

In the future, we might want to refactor this code to inconditionally
use the common Desktop / ES2 subset (like the rest of Qt does),
and then resolve only the functions actually available at runtime.

For now, state why we're doing that on Windows.

Change-Id: Ic21035bcd88ddc1d9274fd90a146c2824d783b25
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Correctly manipulate tm_gmtoff the way qt_timezone() needs.

Follow-up to 91d3298: qt_timezone() expects the number of seconds west
of UTC, whereas tm_gmtoff returns the number of seconds east of UTC, and
contrary to the timezone variable it is not oblivious to DST.

We have to account for those two facts and make sure we return a value
compatible with what timezone would have.

Change-Id: Iacb9077f50d4c847ac09e5a7e952d0e4cd22da1b
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>